This application is a Continuation of International Application PCT/FI01/00277 filed Mar. 20, 2001 which designated the U.S. and was published under PCT Article 21(2) in English.
The invention relates to measuring quantities of an electric line.
In connection with reliable transfer and distribution of electric energy, it is important that the quality of the energy to be transferred can be monitored in a reliable manner. Therefore, quantities of transmission and distribution lines, such as voltages and currents, are measured, which enables e.g. possible failures to be detected and located. A known way to measure the quantities of an electric wire is to arrange measurement units in connection with the phase conductors of an electric line to measure e.g. the current and voltage of the conductors and to transmit, e.g. over a radio path, measurement information to a control unit, which further interprets the measurement information and possibly forwards it. Such measurement arrangements are disclosed e.g. in patent publication U.S. Pat. No. 5,565,783 and published application EP 0 314 850.
When e.g. the summation current of the phases of an electric line, i.e. a zero current, in a multi-phase system is to be determined using phase-conductor-specific phase currents, not only the magnitude of each phase current but also the phase angle, i.e. the phase current phasors, of each phase current have to be known, which enables the phasor of the zero current to be obtained by adding up the phase current phasors. In the prior art measurement arrangements, measurements to be carried out in measurement units of different phases are usually synchronized with each other such that the control unit is able to place the measurement signals of a particular quantity supplied from different measurement units on the same time axis, which enables the phase angles between the phase-specific values of the quantity to be determined by comparing the measurement signals. The measurements can be synchronized e.g. by attaching, at a measurement unit, a time stamp to each measurement result to indicate the point in time at which the measurement was carried out. Another alternative is to control the measurements from the control unit such that the control unit informs the measurement unit about the time of measurement.
The problem with the arrangement described above is that in order to synchronize the measurements, the measurement units must, one way or another, be synchronized with each other and the measurements must be performed simultaneously, which makes the measurement system more complicated to implement. Furthermore, if the measurements are controlled e.g. from the measurement unit and the data transmission between the control unit and the measurement units is based on a radio connection, the radio connection has to be a bidirectional one, which means that the measurement units are thus also provided with receiver equipment.